Research this past year focused on the molecular characterization of glutamate receptors and their function in the auditory system. Four families of ionotropic glutamate receptors and eight metabotropic glutamate receptors have now been identified. In order to characterize the biochemical and cell biological properties of this complex set of proteins, we developed selective antibodies to most of the ionotropic receptor subunits and to a number of the metabotropic receptors. Several studies addressed the biochemical and cell biological properties of glutamate receptors in the central nervous system and cochlea. A number of novel antibodies were developed this past year. We published a study on developing and using antibodies selective for mGluR2 & 3. We developed an antibody selective for GluR2, the subunit of the AMPA receptor family that alone controls calcium permeability of the ion channel. Using this antibody, we can use immunocytochemistry to identify calcium-permeable and -impermeable AMPA receptors. Most neurons express multiple glutamate receptors and several subunits of any ionotropic receptor, and several studies addressed the role of these multiple receptors. Immunoprecipitation analyses suggest that multiple AMPA receptor complexes, which differ in their subunit compositions, are expressed in CA1/CA2 hippocampal neurons. Our studies of the cerebellar Purkinje neuron show that several glutamate receptors are co-expressed at its two synaptic populations: the climbing fiber input and the parallel fiber input. However, the delta subunit is only found postsynaptic of the parallel fiber input in the adult, but associated with both synaptic populations in the young animal. Glutamate receptor distribution was also studied in fusiform cells of the dorsal cochlear nucleus. This neuron receives two different glutamatergic inputs on its apical and basal dendrites. Our studies show that different glutamate receptors are expressed postsynaptic of the two inputs. For example, GluR4, mGluR1a, and mGluR2/3 are found postsynaptic of inputs to basal dendrites but not to those of apical dendrite synapses.